1. Field of the Invention
The present invention relates to infrared detectors, and in particular to an infrared detector with means for continually checking the proper functioning of the detector.
2. Description of the Prior Art
One of the principle demands made on alarm systems, such as fire alarm systems, in which infrared detectors are used, is the requirement that the system and/or the individual components thereof be self-monitoring to a large extent. This requirement is in addition to reliability against false alarms and simple operation of the system. In active alarm systems, self-monitoring of the detectors is easily established. An outage results in an alarm at a reception element. In passive systems, for example, in a unit having an infrared transmitter and an infrared detector, an outage of the detector element is recognized by the electronic circuitry connected thereto when no output signal of the detector element is generated when the transmitter is energized, such as by a pulse generator.
Conventional infrared sensing units generally operate as follows. A pyro-electric detector element is connected to a field effect transistor as a source follower in order to reduce the high output impedance of the detector element, which is on the order of magnitude of 10.sup.13 ohms. In order to be electronically processable an impedance on the order of magnitude of 5K ohms is needed, and is obtained by the aforementioned transistor. In such a circuit, an offset voltage on the order of magnitude of 1 V is established at the signal output of the field effect transistor. If damage to the crystal receiver material or ceramic receiver material occurs, for example, due to broken wire leads, the offset voltage at the output does not change. This is the most frequent cause of error which may occur because improper handling, i.e., dropping or striking of the unit, can easily lead to a breakage of the connecting wires in the unit. For conventional sensing units, the unit can be checked for continuing functionability by a user of the alarm system making a walk-in test, i.e., the user moves within the optical receiving region of the alarm and thus intentionally triggers an alarm signal. If under such circumstances an alarm signal is not triggered, it is assumed that the detector (as the receiving element) or the electronic circuitry, is defective. This method is rather involved and time-consuming, and of course, cannot be undertaken when the user is absent. The continuing functioning of the system, however, must be guaranteed above all when the user is absent.